Feedback suppression of the primary humoral immune response to sheep erythrocytes (SRBC) in vitro was induced with cell-free supernate material derived from antigen-(SRBC) activated B (sIg+) cells. This soluble products bears Ig determinants and binds to the eliciting antigen (SRBC). The activity of this antibody in suppressing anti-SRBC plaque-forming cell responses is restricted to spleen cell cultures containing B cells sharing VH genes with the B cells producing the suppressive antibody. The anti-hapten (trinitrophenyl) response to derivatized SRBC is not affected by antigen-primed B cells or their products. These data are compatible with suppression being mediated by anti-antigen antibody, either (a) via blockade of different SRBC epitopes recognized by a limited set of B cell clones in each mouse strain, (b) via triggering of an anti-idiotypic response, either antibody or suppressor T cell in nature, restricted to activity in cultures containing B cells sharing VH structures with the original antibody, or (c) via interference by preformed antibody with T cell help directed at idiotype bearing B cells.

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